[Distributed] dont crash when concrete system is invalid/missing type

Author: ktoso

test/Distributed/distributed_actor_system_missing_system_type.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-frontend -typecheck -verify -enable-experimental-distributed -disable-availability-checking -I %t 2>&1 %s
+
+// UNSUPPORTED: back_deploy_concurrency
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import Distributed
+
+// This system does not exist: but we should not crash, but just diagnose about it:
+typealias DefaultDistributedActorSystem = ClusterSystem // expected-error{{cannot find type 'ClusterSystem' in scope}}
+
+distributed actor MyActor {
+  // expected-error@-1{{distributed actor 'MyActor' does not declare ActorSystem it can be used with}}
+  // expected-note@-2{{you can provide a module-wide default actor system by declaring:}}
+  // expected-error@-3{{type 'MyActor' does not conform to protocol 'DistributedActor'}}
+
+  distributed var foo: String {
+    return "xyz"
+  }
+}
+
+distributed actor BadSystemActor {
+  // expected-error@-1{{distributed actor 'BadSystemActor' does not declare ActorSystem it can be used with}}
+  // expected-note@-2{{you can provide a module-wide default actor system by declaring:}}
+  // expected-error@-3{{type 'BadSystemActor' does not conform to protocol 'DistributedActor'}}
+
+  // This system does not exist: but we should not crash, but just diagnose about it:
+  typealias ActorSystem = ClusterSystem // expected-error{{cannot find type 'ClusterSystem' in scope}}
+}